AD5228 Data Sheet
Rev. B | Page 12 of 18
If the
PU
button is held for longer than 1 second, continuously
holding it activates autoscan mode such that the AD5228
increments by four R
WB
steps per second (see Figure 3).
Whenever the maximum R
WB
(= R
AB
) is reached, R
WB
stops
incrementing regardless of the state of the
PU
pin. Any continu-
ous holding of the
PU
pin to logic-low simply elevates the supply
current.
When both
PU
and
PD
buttons are pressed, R
WB
decrements
until it stops at zero scale.
All the preceding descriptions apply to
PD
operation. Due to
the tolerance of the internal RC oscillator, all the timing
information given previously is based on the typical values,
which can vary ±30%.
The AD5228 debouncer is carefully designed to handle common
pushbutton tactile switches. Other switches that have excessive
bounces and duration are not suitable to use in conjunction
with the AD5228.
04422-0-035
B
W
A
D0
D2
D1
D4
D3
R
S
R
S
=
R
AB
/32
R
W
R
S
R
S
R
S
RDAC
UP/DOWN
CTRL AND
DECODE
Figure 34. AD5228 Equivalent RDAC Circuit
PROGRAMMING THE DIGITAL POTENTIOMETERS
Rheostat Operation
If only the W-to-B or W-to-A terminals are used as variable
resistors, the unused terminal can be opened or shorted with W.
Such operation is called rheostat mode and is shown in Figure 35.
04422-0-036
A
W
B
A
W
B
A
W
B
Figure 35. Rheostat Mode Configuration
The end-to-end resistance, R
AB
, has 32 contact points accessed
by the wiper terminal, plus the B terminal contact if R
WB
is used.
Pushing the
PU
pin discretely increments R
WB
by one step. The
total resistance becomes R
S
+ R
W
as shown in Figure 34. The
change of R
WB
can be determined by the number of discrete
PU
executions provided that its maximum setting is not reached
during operation. ∆R
WB
can, therefore, be approximated as
W
AB
WB
R
R
PUR
32
(1)
W
AB
WB
R
R
PDR
32
(2)
where:
PU
is the number of push-up executions.
PD
is the number of push-down executions.
R
AB
is the end-to-end resistance.
R
W
is the wiper resistance contributed by the on-resistance of
the internal switch.
Similar to the mechanical potentiometer, the resistance of the
RDAC between the Wiper W and Terminal A also produces a
complementary resistance, R
WA
. When these terminals are used,
the B terminal can be opened or shorted to W. R
WA
can also be
approximated if its maximum and minimum settings are not
reached.
W
AB
WA
R
R
PUR
32
32
3)
W
AB
WA
R
R
PDR
32
32
(4)
Note that Equations 1 to 4 do not apply when
PU
and
PD
= 0
execution.
Because in the lowest end of the resistor string, a finite wiper
resistance is present, care should be taken to limit the current
flow between W and B in this state to a maximum pulse current
of no more than 20 mA. Otherwise, degradation or possible
destruction of the internal switches can occur.
The typical distribution of the resistance tolerance from device
to device is process lot dependent, and ±20% tolerance is possible.
